This invention relates to electrophoretic fluids, the use of these fluids for the preparation of an electrophoretic display device, and electrophoretic displays comprising such fluids.
In recent years a need has developed for low power, low cost and light weight display devices. EPDs (Electrophoretic Displays) can fulfil this requirement. One use of EPDs is for electronic paper. It is imperative that once an image is displayed, the image can be retained for a long period of time without further voltage being applied. Hence, this fulfils the requirements of low power use, and means an image can be visible until another image is required.
An EPD generally comprises charged electrophoretic particles dispersed between two substrates, each comprising one or more electrodes. The space between the electrodes is filled with a dispersion medium which is a different colour from the colour of the particles. If a voltage is applied between the electrodes, charged particles move to the electrode of opposite polarity. The particles can cover the observer's side electrode, so that a colour identical to the colour of the particles is displayed when an image is observed from the observer's side. Any image can be observed using a multiplicity of pixels.
Available technologies of EPDs include electronic paper, commercially used in electronic books. This application uses black and white colour. The use of different coloured particles in a single pixel has been exemplified in recent patent literature (U.S. Pat. No. 7,304,634, GB 2 438 436, US 2007/0268244). WO 2010/089057, WO 2010/089058, WO 2010/089059, and WO 2010/089060 describe coloured polymer particles for EPD.
Particles suitable for use in electrophoretic displays (EPD), e.g. coloured electronic paper have been exemplified in recent patent literature; e.g. (U.S. Pat. No. 7,304,634, GB 2 438 436, US 2007/0268244). It is now possible to control and manipulate features such as size, colour, charge and polydispersity independently to produce particles with desired properties for EPD; e.g. WO 2010089057. However, the colour properties are dictated by either dye chromophores or pigment shade as used in a particle. The concept of colour mixing is well known and understood. Typically, RGB or CMY colours are mixed to give other colours, for example, cyan and yellow mix to give green, and so on. This concept has been applied to EPD in terms of pixellated displays, or even in terms of mixing dyes prior to particle synthesis.
There is a requirement for colour particles of specific colour coordinates, which can be easily dispersed in non-polar media, show electrophoretic mobility and which do not leach colour in a dispersant. To obtain a desired colour coordinate for a particle, the dye chromophores structure can be changed, or an alternative pigment found—however these solutions can be expensive, complex, and time consuming. In addition, some colours are difficult/impossible to achieve with a single dye chromophore and dye blending prior to synthesis causes complications with controlling dye incorporation and can require lengthy and expensive synthesis programs. There is a desire for electrophoretic display materials with improved colour tuning, for example to match a company logo colour, to enhance colour gamut, or to improve contrast ratio. Therefore, the object of this invention is to provide new electrophoretic display materials.